The present invention relates to fluid coupling devices and to friction coupling devices that drive radiator cooling fans. Such coupling devices are generally of two types, viscous clutch drives and dry friction clutch drives, respectively. Viscous drives have varying degrees of engagement and are used for their ability to cycle repeat and engage at higher engine speeds. Dry friction drives are used for their ability to turn at fully engaged peak operating speeds or at higher speeds than originally designed, as well as for their relatively cool operating temperature. Dry friction drives are also generally less complex.
Dry friction drives tend to have two operating conditions “ON and OFF” referring to when a friction clutch is either fully engaged or fully disengaged. When a friction clutch assembly is fully engaged the clutch assembly provides cooling to an associated engine and is not slipping. When a friction clutch assembly is fully disengaged, slip speed between a clutch plate and an engagement surface is at a maximum and cooling is no longer provided.
During the actuation of a dry friction clutch between full engagement and full disengagement, the internal air volume within the clutch assembly changes. This change in volume creates a change in internal pressure. As a result of this change in internal pressure, air passes between a small clearance gap between a spring carrier and a pneumatic transfer conduit or piston rod. Over time this clearance gap or exhaust gap can become blocked, due to internal contaminates and grease or fretting corrosion between the spring carrier and the piston rod, which can build up and block the flow of air.
The phenomena of blocking the exhaust gap can cause contaminants to be forced through bearings in the fan clutch. The blocking of the airflow between the spring carrier and the piston rod forces the air to travel an alternate route, such as through the grease seal of a proximate bearing. This alternative route is undesirable, due to potential contamination of the bearing internals. Forcing of the air through clutch bearings degrades and reduces the service life of the bearings and thus reduces the service life of the clutch assembly.
The application of grease to the interface between the spring carrier and the piston rod can temporarily prevent the fretting corrosion. However, due to the small size of the clearance gap deposits can still build up and block the airflow. In addition, the applied grease can block the airflow and increase the flow of air through the clutch bearings. Also, the opening or the increasing of the separation distance of the interface between the spring carrier and the piston rod is not feasible, as the tight radial fit therebetween is desired for proper clutch operation.
There is a desire to increase fan clutch service life such that a fan clutch is capable of being operated over increasingly higher mileages. Thus, there is a need for an improved friction clutch assembly that accounts for the changes in internal air pressure and provides the desired clearances between clutch components for proper operation thereof. It is also desirable that the improved friction clutch assembly be similar in complexity and cost to that of traditional friction fan drive systems.